A typical plunger filter apparatus of the kind under discussion comprises a vessel and a plunger assembly including a filter element. The vessel is usually of glass having a cylindrical side wall and a bottom wall. The plunger assembly comprises a perforated disc, a filter element, a spider plate and a plunger shaft. The perforated disc is adapted at its periphery to slidably and sealingly engage the cylindrical side wall of the vessel and acts as a support for the filter element. The filter element is for example a mesh which is clamped between the disc and the spider plate and is of a mesh size which permits passage of liquid but substantially prevents passage of coffee grounds. The spider plate has an inner ring, an outer ring and for example three legs extending between the rings and acts as an upper retaining member for the filter element so that in use the filter is not excessively distorted or weakened. The shaft extends axially from outside the vessel successively through apertures in the lid, spider plate, filter element and threadably engages a boss of the perforated supporting disc. As the shaft is screwed into the boss, a shoulder of the shaft exerts a clamping force on the filter element between the upper spider plate and lower perforated disk.
In normal use, with the lid and plunger filter removed, ground coffee is first spooned into the vessel, boiling water is then added, and the plunger assembly is next inserted into the vessel with the filter element above the contents and the lid closing the vessel. Alternatively, the water is boiled in the glass vessel and then coffee added. The apparatus is then allowed to stand while the coffee brews. When the coffee is sufficiently brewed the plunger shaft is depressed pushing the filter mesh through the liquid whereby filtered liquid passes to the upper side of the filter and coffee grounds are pushed towards the bottom wall of the vessel. Filtered coffee can then be poured from the vessel via a lip, the coffee grounds remaining trapped between the filter element and the vessel bottom wall.
Apparatus of that kind is available in various forms. The filter element may be a plastic or wire mesh or finely perforated metal plate or the like. The plunger parts are usually adapted for easy disassembly for cleaning and reassembly and may be made in a variety of materials and shapes.
Although well-known, apparatus of the above described kind suffers from a number of deficiencies.
When water is added to ground coffee, some of the grounds tend to float forming a plug adjacent the upper surface of the liquid while others fall to the bottom. Diffusion of hot water into the plug is slow and extraction occurs mainly at the lower surface of the plug and at the bottom of the vessel. Furthermore the force required during the filtration step to move the plunger and push the plug of coffee grounds downwards is great and occassionally breaks the vessel when made of glass.
These disadvantages can be alleviated by stirring the coffee grounds with a spoon after adding the water but prior to inserting the plunger assembly. However, in that case there is a loss of flavour and aroma from the surface to the surrounding atmosphere while stirring takes place and the vessel is uncovered. For this or other reasons most consumers do not stir prior to replacing the plunger assembly and lid.
A further disadvantage which is not alleviated by stirring is that the grounds which subsequently fall to the bottom (and also those subsequently pressed to the bottom by the filter element) are swollen with water which contains concentrated flavour components and which are not readily extractable in conventional plunger apparatus because flavour components remain in the interstices of the coffee grounds.